A popular tape recorder apparatus for recording audio signals on, and reproducing audio signals from, a magnetic tape is the rotary head digital audio tape (R-DAT) apparatus. The general format of the R-DAT apparatus is described in THE DAT CONFERENCE, THE DAT CONFERENCE STANDARD, Mar., 1988. U.S. Pat. No. 4,841,390 and Japan Patent Doc. 59-231713 also describe R-DAT apparatuses. A R-DAT for use as a data storage device, e.g., in a computer system, has also been proposed by K. Odaka, E. Tan & B. Vermeulen, "Designing a Data Storage Format for Digital Audio Tape (DAT)", rev. B, Oct., 1988. See also European Patent Application Nos. 0 272 130, 0 314 456.
FIG. 1 shows a computer system 10 which utilizes a R-DAT 22 for data storage. The computer system 10 has a CPU or processor 12 for executing instructions, a main memory 14 and a disk memory 16 for storing data and instructions, a keyboard 18 for receiving manual input of data and instructions, a display device 20, a R-DAT 22 and a bus 24 for transferring data and instructions between each device. Illustratively, the R-DAT 22 is used as a data streamer, e.g., the R-DAT may be periodically used for archiving or backing up the disk memory 16.
In a R-DAT, signals are recorded in digital form on a magnetic tape. As shown in FIG. 2, tracks 2 are formed on the magnetic tape 1 obliquely to the longitudinal direction of the tape 1. Typically, the tracks 2 are formed alternately by a pair of recording heads A and B of different azimuth angles. (The heads A and B are positioned 180.degree. from each other on the outer circumference of a rotating drum around which the tape is partially wrapped.) Each pair of "plus-azimuth" and "minus-azimuth" tracks forms a frame. As shown, each track is divided into a PCM area containing one hundred twenty-eight blocks of recorded data, SUB1 and SUB2 data areas containing eight blocks of PACK data each and ATF (automatic track following) areas containing signals used for tracking servo control during playback.
As shown in FIG. 2A, each PCM block includes bytes W1, W2 and W3 where W1 and W2 are block ID codes and where W3 is a parity byte (equal to the bit-wise W1 exclusive-OR W2). Each PCM block also has a sync byte and thirty-two PCM data bytes. Likewise, as shown in FIG. 2B, each SUB block includes bytes W1, W2 and W3 where W1 and W2 are block ID codes and where W3 is a parity byte (equal to the bit-wise W1 exclusive-OR W2). Each SUB block also has a sync byte and thirty-two PACK data bytes.
The-recording of data on, and the reproduction of data from, the magnetic tape 1 is subject to error including drop-out and drop-in errors. A drop-out error occurs when the signal level of data recorded on the tape 1 has degraded such that the data can no longer be reproduced. A drop-in error occurs when an old tape on which old data bytes were previously recorded is used for recording new data. FIG. 3 illustrates how a drop-in error occurs. A conventional R-DAT does not have an erase head. Rather, new data bytes are simply recorded over old data. As shown in FIG. 3, new data bytes are about to be recorded over old data 3 on a track 4. However, because of some event or condition, such as a dust particle being present between the recording head which is about to scan the track 4 for purposes of recording the new data and the tape 1, the new data bytes do not overwrite the old data 3. Thus, the old data bytes 3 remain on the track 4.
A R-DAT for use in recording and reproducing signals provides several error detection and correction mechanisms including:
(a) Interleaving--In ordinary audio recording, the left and right channels of every thirty msec of audio are recorded in a single frame comprising a pair of tracks--a plus-azimuth track and a minus-azimuth track. According to an interleaving scheme, each byte (sample of the left or right channel) within a frame is recorded in the PCM data areas of the tracks of the frame such that adjacent samples are separated from each other as much as possible. FIGS. 4A and 4B show illustrative interleaving formats for the plus-azimuth track and the minus-azimuth track. In FIGS. 4A and 4B, the block Aiu is an upper byte of the ith sample of the left channel, Ail is the lower byte of the ith sample of the left channel, Biu is the upper byte of the ith sample of the right channel and Bil is the lower byte of the ith sample of the right channel. Px,y and Qx,y are parity symbols of error correction codes (ECC) C1(32,28,5) and C2(32,24,7) discussed below. PA0 (b) Parity check--Illustratively, a digital signal to be recorded is divided into a series of eight bit long units which are modulated to produce ten bit long units. The eight bit long units are recorded in thirty-two byte blocks which also include W1 (ID1), W2 (ID2) and W3 (=W1 XOR W2) bytes (described above). When the signal is reproduced from the tape, the eight bit of data are demodulated and a parity check is performed on the entire block using the block ID codes W1 and W2 and the parity check byte W3. That is, the R-DAT verifies for the reproduced bytes W1, W2 and W3 that W3=W1 XOR W2. PA0 (c) Error Correction Codes (ECC)--ECC encoding is performed on PACK and PCM data during the recording process using a Reed Solomon code. In the case that the R-DAT is used for audio, two ECC codes C1 (32,28,5) and C2 (32,26,7) are used. If the R-DAT is used for data storage, a third code C3 is used. The ECC provides for detection and correction of errors provided that the number of errors is within the error detection and error correction capabilities of the ECC codes C1, C2, and C3. PA0 (d) Read after write (RAW)--A R-DAT used for data storage is adapted (as discussed below) to reproduce each track immediately after it is recorded. If the reproduced data bytes contains errors, they are re-recorded on the tape.
The interleaving format facilitates the "concealment" operation which reconstructs bytes lost to drop-out or burst errors (an error in which adjacent bytes recorded on a track are lost). In a concealment operation, lost data bytes are reconstructed from one or more nearby recovered data using interpolation, smoothing or holding the previous sample. By dispersing adjacent samples as far away as possible, the likelihood increases that lost data bytes will not be adjacent to each other but instead will be adjacent to reproducible data bytes in the reconstructed signal (which adjacent reproducible data bytes may be used in the concealment operation to recover the lost data bytes).
As can be appreciated from the above discussion, more error correction and detection capabilities are provided for a R-DAT adapted for data storage than a R-DAT used for audio. The reason is that in audio reproduction, errors not corrected or detected using the above mechanisms can be alleviated using a concealment operation. This is not possible in the case of data storage where absolutely accurate data reproduction is critical.
FIG. 5 illustrates a conventional R-DAT 300 adapted for data storage. The R-DAT 300 has a timing generator 304 for synchronizing the operation of the R-DAT 300. As shown, the R-DAT 300 has four heads, two recording heads Ar, Br and two playback heads Ap, Bp. Each playback head Ap or Bp is positioned on the outer circumference of a rotating drum 301 270.degree. (in the direction of rotation of the drum 301) from a respective recording head Ar or Br. During ordinary reproduction, a signal is reproduced from a track on a magnetic tape 320 partially wrapped around the rotating drum 301 via one of the playback heads Ap or Bp. The playback heads Ap and Bp convert magnetic signals on the tape to electrical signals which are amplified in the amplifiers 313 and 310. The amplified signal outputted from the amplifier 310 is inputted to an ATF circuit 308. The ATF circuit 308 outputs a tracking error signal to a capstan servo 303. The capstan servo 303, in turn controls the tracking in response to the tracking error signal. The capstan servo 303 controls a capstan motor in accordance with the frequency and phase information feedback from the capstan motor. Likewise, the drum servo 302 controls a drum motor in accordance with frequency and phase information feedback from the drum motor.
The signal outputted from the amplifier 313 is inputted to an equalizer 314. The signal outputted from the equalizer 314 is inputted to a PLL circuit 315. A signal outputted from the PLL circuit 315 and the signal outputted from the equalizer 314 are inputted to a demodulator circuit 316. The signal outputted from the equalizer 314 is organized into ten bit units. The demodulator circuit 316 demodulates the signal into eight bit units and performs a-parity check on the demodulated data. If thee demodulated data bytes pass the parity check, they are loaded into RAM bank 307.
Thereafter, the ECC circuit 306 performs error detection and correction on the data in the RAM bank 307. Afterwards, the PACK data bytes are outputted via the subcode interface 309 to a host interface 318. Illustratively, the host interface 318 interconnects the R-DAT 300 with a computer (such as the computer 10 of FIG. 1). In addition, the PCM data byte are inputted to a PCM interface 305 where the data bytes are de-interleaved. The de-interleaved PCM data bytes are then outputted to the host interface 318.
Illustratively, the R-DAT 300 records data using a RAW procedure. First, PCM data and PACK data from the host interface 318 are inputted to the PCM interface 305 and subcode interface 309, respectively. The PCM data bytes are interleaved in the PCM interface 305. The interleaved PCM data bytes and PACK data bytes are then combined and stored in the RAM bank 307. The combined data bytes are then ECC encoded by the ECC circuit 306. The encoded data (in eight bit form) are modulated into ten bit data units in the modulator 311 using an 8 to 10 conversion table. The signal in ten bit form is then outputted via the amplifier 312 to one of the recording heads Ar or Br. The recording head Ar or Br records a magnetic signal representing the data signal on the magnetic tape.
In the RAW process, after data bytes are recorded, the data bytes are reproduced for purposes of verifying that they were properly recorded. This is achieved as follows. Referring to FIG. 6, suppose the recording head Ar records a track of data A.sub.n on the tape. Some time T after the recording head Ar scans the track A.sub.n for purposes of recording, the playback head Ap scans the same track A.sub.n and reproduces the data stored thereon. As shown, the playback head Ap is positioned 270.degree. of one rotation of the drum 301 from the recording head Ar and therefore scans the track A.sub.n immediately before the playback head Ap scans the next track A.sub.n+1.
The reproduced data bytes are fed to the demodulator circuit 316 as before in ordinary reproduction. Therein, the demodulator circuit 316 performs a parity check on the data. Furthermore, the ECC circuit 306 performs an ECC check on the data. If the data bytes fail either check, the data bytes are re-recorded on the tape. Otherwise, the data bytes are considered valid and the recording head Ar records the next data (which is loaded in the RAM bank 307 and ECC encoded in parallel with reproducing the data from track A.sub.n) on track A.sub.n+1 of the tape.
The prior art RAW process of the R-DAT 300 is disadvantageous. The ECC and parity checks provide crude methods of error detection. Furthermore, the RAW process increases the load on the ECC circuit 306. Thus, an ECC circuit 306 must be provided with sufficient speed to both encode data to be recorded on one track and decode data reproduced from a previous track for verification purposes within the limited time constraints of the operation of the R-DAT. Moreover, even ECC decoding may fail to detect drop-in errors. This is because, the old pre-existing data which the R-DAT failed to record over may be error free despite being the incorrect data.
European Patent Application 0 297 809 discloses a non-RAW prior art process for detecting drop-in errors due to head clogging in a R-DAT used for data-storage. In this process, a portion of the PCM data of each track is allocated for storing header information. Each frame of data is assigned header information. When tracks of data are reproduced, the header information of each pair of reproduced tracks of each frame is compared. If the header information is not identical, a drop-in error is detected. This process provides limited drop-in error detection for detecting a single track which was not recorded over. However, little or no protection is provided if only a portion of a track (e.g., a SUB area) or a pair of tracks is not recorded over.
It is therefore an object of the present invention to overcome the disadvantages of the prior art.